This invention relates in general to a reaction with distillation column in which mass transfer and chemical reaction occur within the same general region within the column, and, more particularly, to a structure within the column which allows such mass transfer and chemical reaction to occur. The invention also relates to a process employing a plurality of such structures.
Various structures have been suggested for use in processes wherein concurrent reaction with distillation of fluid streams is desired. One such type of reaction with distillation structure employs a distillation tray and a downcomer which extends between adjacent distillation trays and is packed with catalyst particles to form a catalyst bed. The distillation tray facilitates mass transfer between the liquid and vapor streams while the catalyst bed in the downcomer causes the liquid stream to undergo catalytic reaction as it descends between trays. In the design of this type of structure as illustrated in Haunschild U.S. Pat. Nos. 3,629,478 and 3,634,535, all of the downwardly flowing liquid stream is channeled through the downcomer as it descends between adjacent trays.
Forcing all of the liquid to flow through the packed downcomer in the structure described above can be advantageous in certain applications because liquid feed is continually presented to the catalyst and the reaction product is removed from the catalyst surface at a rate fast enough to ensure that the reaction product does not inhibit the effectiveness of the catalyst. In many applications, however, it may be unnecessary to force all of the descending liquid through the packed downcomer because the desired chemical reaction can be accomplished by bringing only a portion of the liquid stream into contact with the catalyst. In such applications, the use of the packed downcomer structure as described may be undesirable because the liquid flow rate that can be achieved in the column is limited by the permeability of the catalyst bed.
Another type of conventional catalytic reaction with distillation structure employs a cloth belt which is supported by a steel wire support structure and has a plurality of pockets filled with catalyst. Smith U.S. Pat. Nos. 4,307,254 and 4,302,356 provide examples of structures of this type. The hydraulic characteristics of a reaction with distillation process which employs the cloth belt can be much higher than when the previously described packed downcomer is used because all of the descending liquid stream is not forced to flow through the catalyst pockets. Instead, the liquid stream is free to flow through the open areas surrounding the cloth belt.
The catalyst in the cloth belt structure previously described is wetted by the liquid soaking through the cloth covering and diffusing through the catalyst. Because the liquid is not forced through the catalyst in the cloth belt structure, the reaction products may in some instances be removed too slowly from contact with the catalyst. The catalyst effectiveness or reactivity in the cloth belt may thus be less than desired in certain applications.
In other applications in which the cloth belt is used, the column region containing the cloth belt may be flooded with liquid to enhance the catalytic reaction of the liquid. Because of the flooded conditions in that portion of the column, mass transfer between the liquid and vapor streams is substantially impeded. Provisions must then be made for allowing distillation to occur elsewhere within the column and the vapor stream must be bypassed around the flooded region. The cloth belt in those applications thus functions primarily as a reaction structure and not a combination reaction with distillation structure.
A still further example of a conventional reaction with distillation structure employs a packing such as in the form of corrugated plates which have a catalyst bed formed between pairs of adjacent plates. Multiple pairs of plates are then arrayed in alternating directions to form liquid and vapor flow channels in the troughs of the plates. This "sandwich" type construction can provide very high mass transfer rates because it causes more uniform distribution of and contact between the liquid and vapor streams. Hydraulic performance can also be very high because the flow channels allow most of the liquid stream to bypass the catalyst. In addition to the favorable mass transfer and hydraulic characteristics, the sandwich construction provides increased catalyst effectiveness because a portion of the liquid stream is forced through the catalyst beds at a rate which facilitates removal of the reaction product from contact with the catalyst.
Although the sandwich construction can provide better processing performance than either the downcomer or cloth belt structures previously described, it can also be much more expensive than those structures and economic considerations may prevent the use of the sandwich construction in many applications. A need has thus arisen for a more economical reaction with distillation structure that provides the desired reactivity as well as mass transfer and hydraulic performance so that the structure can be used in a greater range of chemical processing applications.